1. Field of the Invention
This invention relates generally to additives for modifying suspension properties of drilling fluids. More specifically, this invention relates to solids control properties of drilling fluids used in wells drilled under extreme conditions, such as horizontal or deviated wells, or wells with large boreholes. In particular, this invention relates to the use of a carrageenan, such as iota or kappa carrageenan, as a suspending agent in a drilling mud system.
2. Description of Related Art
The control of solids is of vital concern during drilling and workover operations. Such solids typically include fluid weighting agents and formation solids acquired while drilling. For example, drilling mud is typically a mixture of refined clays, such as bentonite, and water. Fluid weighting agents, such as barite, and other chemicals for varying suspension and Theological properties of a fluid are also typically present in drilling mud.
Drilling mud and other drilling fluids are employed to serve many functions. For example, during drilling operations, drilling mud is typically pumped or injected down a drill pipe string and through a drilling bit. The drilling mud is then circulated upward in the annular space between the walls of the borehole and the pipe string. The hydrostatic pressure of drilling mud maintains borehole stability by preventing the collapse of unstable formations into the hole and preventing the intrusion of fluids from exposed formation strata. At the same time, the circulated mud continuously flushes formation cuttings from around the drill bit and removes these cuttings from the borehole by conveying them upwards toward the surface in the annular drilling space. A similar function may be performed using drilling mud or other types of drilling fluids during remedial operations, such as workovers and recompletions. In some cases, a drilling fluid may be reverse circulated by injecting the fluid down a wellbore annulus and circulating the fluid up an inner pipe string.
To ensure adequate removal of solids, a drilling fluid should have a sufficient viscosity and bearing capacity such that formation cuttings or other solids are suspended in the fluid. Moreover, it is often desirable that a drilling mud be capable of suspending solids even when the mud is stationary. During drilling operations, a drilling fluid having inadequate solids control or suspension characteristics will typically result in a poor rate of drilling penetration and/or may result in the buildup or deposition of solids within the borehole.
In the drilling of vertical or near vertical boreholes, upward flow of drilling mud in the annular drilling space typically tends to be uniform and laminar. This type of flow is conducive to removal and transportation of formation cuttings through the annular drilling space to the surface. To further ensure complete removal of cuttings and to prevent particle fallback, the carrying capacity of drilling muds is often increased by increasing the viscosity of the mud to a point that formation cuttings are held in near suspension in the mud when the mud flow rate is at or near stationary.
When drilling or conducting workover operations in highly directional or horizontal wells, upward flow of mud in the wellbore tends to be less uniform than in more vertically oriented wells. As a result, particle fallback is more difficult to control, and a layer of solids may accumulate on the low side (bottom) of deviated or horizontal sections of a wellbore. Accumulation of particles and cuttings within a well may be greatest in regions of inclination changes. During drilling, problems associated with accumulation of solid particles or cuttings include reduction in particle removal and associated increases in friction and decreases in rate of penetration. In extreme cases, accumulations of solid particles may result in sticking of the bit and/or pipe string. Once formed, a layer of solids or cuttings is typically not easily removed by hydraulic or mechanical means. In addition, obtaining zonal isolation with cement in the presence of a layer of drilling solids is difficult because such a layer interferes with formation of a pressure tight structure. This may result in a loss of hydraulic integrity due to fluid movement through the layer of drilling solids deposited underneath the cement sheath of a completed well.
Solids accumulation problems may also be encountered when drilling boreholes of relatively large diameter, such as those having a diameter greater than about 16 inches. These wells usually traverse shallow and poorly consolidated formations and are typically drilled using high rates of penetration. Drilling of large diameter boreholes is further typically complicated by a large volume of cuttings that must be removed from the borehole and by large volumes of drilling fluid and relatively high fluid circulation rates to suspend and remove these cuttings. In some cases, water is used as a drilling fluid to save expense and storage space associated with large volumes of drilling mud containing clays such as bentonite. However use of water in this context requires high circulation rates that may result in leaching of borehole walls. In addition, cuttings and other solids typically settle out when drilling and fluid circulation is stopped, resulting in drilling difficulties when drilling and circulation is reinitiated.
In the past, one solution for addressing solids accumulation and removal problems during operations on highly deviated wells has been to vary the flow conditions of a drilling fluid. For example, fixed volume or "spacers" of drilling mud having viscosities less than or greater than the bulk of a drilling mud may be periodically pumped into a well to create turbulence or plug flow within the drilling space annulus. However, this solution complicates the drilling procedure and may be only partially effective.
In other cases, oil based or oil emulsion drilling fluids have been employed to increase the carrying capacity and lubricity of a drilling system. However, these muds are expensive and typically present environmental problems.